fix(wasm_backend): remove buggy local .cargo/config.toml shadowing root

The local config at src/sdk/python/wasm_backend/.cargo/config.toml has
been present since the initial commit (b0c7399). It set:

    [env]
    WIT_WORLD = { value = "../../../../wasm_sandbox/wit/sandbox-world.wasm",
                  relative = true }

Cargo resolves `relative = true` against the directory containing the
`.cargo/` directory (the workspace member root, not the `.cargo/` dir
itself). Counting four `..` from src/sdk/python/wasm_backend/ lands at
the *parent* of the repo root, then `wasm_sandbox/...` -> a path that
does not exist. The real artifact lives at src/wasm_sandbox/... so the
correct prefix would have been `../../../` (three `..`), not four.

Because cargo merges configs deepest-first and this local file shadowed
the root .cargo/config.toml (which is correctly written), every cargo
invocation made from within src/sdk/python/wasm_backend/ -- including
`maturin build` invoked by `pip install` -- saw WIT_WORLD pointing at a
non-existent path. The hyperlight_wasm_macro::wasm_guest_bindgen proc
macro in hyperlight-wasm-runtime then panicked with:

    proc macro panicked: called `Result::unwrap()` on an `Err` value:
    Os { code: 3, kind: NotFound, message: "The system cannot find the
    path specified." }

This was masked everywhere it should have surfaced because every
`just` recipe that exercises this code path exports WIT_WORLD as an
absolute path before invoking cargo, bypassing the buggy resolution.

The repo-root .cargo/config.toml (added previously) already covers
every cargo invocation that walks up from inside the workspace, so the
local config is redundant. Remove it to eliminate the shadow and let
the correctly-resolved root config take effect.

Verified empirically: in a fresh clone of feat/scoped-credentials at
a853591, `cargo build --lib --release` from src/sdk/python/wasm_backend/
with no env vars set previously panicked at the proc-macro; after
removing this file (the only change), the same command finishes the
release build in ~1m18s.

Signed-off-by: Simon Davies <simongdavies@users.noreply.github.com>

Author: simongdavies